Friend of yours dating
Whether you're gay, straight, bi, or not into labels, dating a friend's ex can absolutely be done without sacrificing your friendship — you just have to follow a few simple guidelines. (I'm going to use female pronouns for your friend, and male pronouns for your sweetie, for the sake of simplicity; however, every rule here applies no matter the genders of the participants.) Keep your friend's secrets.
The reverse is also true; no matter how much you love discussing your dude with your besties, his ex can probably live without hearing the details of his current sex life. It's OK to come to your partner for advice if you're arguing with your friend, or vice versa, but absolutely resist the urge to belittle or insult one of them to the other.
In fact, when we met, my now-partner was on a date with my best friend.
However, in order to maintain a healthy relationship with both of them, it's crucial that you never seem even a little like you're taking sides in their breakup or casting either one as the bad guy, even months or years after the fact. For instance, if your friend doesn't want to go to parties where her ex will be in attendance, don't pressure her.This has nothing to do with some kind of Eternal Dibs situation, and everything to do with the fact that, by choosing to build a relationship with someone who treated her horribly, you're telling your friend you don't think what he did to her was all that bad. There are lots of people out there who are just as good in bed and haven't traumatized anyone you care about.